The present invention relates to alkyl-substituted diisocyanates of the diphenyl methane series having isocyanate groups in the 3,4' position and to processes for the production of such alkyl-substituted diisocyanates.
The organic polyisocyanates which are technically and economically important as starting materials in the production of polyurethane plastics include 2,4-diisocyanato-toluene and mixtures thereof with 2,6-diisocyanato-toluene (TDI), 4,4'-diisocyanato-diphenyl methane and mixtures thereof with 2,2'- or 2,4'-diisocyanato-diphenyl methane and/or with homologues of higher functionality (MDI). Although these aromatic polyisocyanates are universally used in large quantities for the production of polyurethane plastics (particularly foams and elastomers) they are attended by certain disadvantages. TDI, for example, has a high vapor pressure and for physiological reasons requires strict safety measures in processing. MDI, particularly 4,4'-diisocyanato-diphenyl methane which is generally present as the main component in polyisocyanate mixtures of the diphenyl methane series tends to crystallize and is a solid at room temperature. Therefore, MDI raw materials often have to be liquefied before processing. This liquification is accomplished either by heating the MDI material to a temperature above the melting point of 4,4'-diisocyanato-diphenyl methane or by chemical modification such as partial urethanization (See for example U.S. Pat. No. 3,644,457) or partial carbodiimidization (See for example U.S. Pat. No. 3,152,162).
It would therefore be advantageous to have aromatic polyisocyanates which could be used in making polyurethanes having properties comparable to those made from TDI and MDI but which need not be liquified before use and are not as physiologically hazardous as these prior art materials.